A Fischer-Tropsch process comprises the hydrogenation of CO in the presence of a catalyst based on Group VIII metals, such as Fe, Co and Ru. The products formed from this reaction are gaseous, liquid and waxy hydrocarbons which may be saturated or unsaturated. Oxygenates of the hydrocarbons such as alcohols, acids and aldehydes are also formed. The carbon number distribution of the products follow the well-known Anderson-Schulz-Flory distribution.
A heterogeneous Fisher-Tropsch process may be conveniently categorised as either a high temperature Fischer-Tropsch (HTFT) process or a low temperature Fischer-Tropsch (LTFT) process. The HTFT process can be described as a two-phase Fischer-Tropsch process. It is usually carried out at a temperature from 250° C. to 400° C. and the catalyst employed is usually an iron-based catalyst, usually a fused iron catalyst. At the temperatures used for this process both the reactants and the products are in a gas phase in the reaction zone, and the catalyst, which is a solid, forms the second phase. Generally the process is commercially carried out in a fluidised bed reactor and the products obtained are of higher olefinicity and shorter chain length (that is products in the gasoline and diesel range) compared to the products of the LTFT process.
The LTFT process can be described as a three-phase Fischer-Tropsch process. It is usually carried out at a temperature from 240° C. to 310° C. and the catalyst employed is usually a Co-based catalyst, but it can also be a Fe-based catalyst. The conditions under which this process is carried out, results in the products being in a liquid phase in the reactor. Therefore this process can be described as a three-phase process, where the reactants are in the gas phase, the products are in the liquid phase and the catalyst is solid in the reaction zone. Generally this process is commercially carried out in a fixed bed reactor or a slurry bed reactor. The products from this process are heavier hydrocarbons such as waxes. A fluidised bed reactor cannot be used in the LTFT process, as the liquid product will cause adhesion of the solid catalyst particles, which will affect the fluidisation properties of the catalyst.
Since the HTFT and LTFT processes are different, the catalyst that is used in each of the processes will be different. The catalyst is generally optimised for a specific process and for the attainment of a specific range of products.
As stated above, the catalyst which is commonly used in the HTFT process is a fused iron catalyst and this catalyst is promoted, usually with a source of alkali or alkaline earth metals. Fused catalysts have a high mechanical strength which is required due to the robust conditions in a fluidised bed where rapid mixing of the two phases takes place at a high temperature.
Fused iron catalysts are usually prepared from low impurity iron sources, e.g. mill scale. The process for preparing a fused iron catalyst usually entails mill scale from a steelwork being fused together with desired amounts of promoters to obtain molted iron. The molten iron is cast into ingots and the latter is crushed and then milled in a ball mill to the required particle size. A major disadvantage is that the supply is dependent on the throughput of steelworks and the impurity levels in the mill scale are not always consistent, which has a negative influence on the catalyst performance.
Another type of catalyst used in Fischer-Tropsch processes is a precipitated catalyst. In these catalysts improved control over impurity levels can be obtained, but they have always suffered from the disadvantage that precipitated catalyst particles are not sufficiently robust to be used in fluidised bed reactors of the HTFT process. Accordingly, precipitated catalysts have usually only been used in LTFT processes. However, PCT/ZA01/00084, filed by the same applicant of the present application, discloses an iron based precipitated catalyst of sufficient mechanical strength to be used in a fluidised bed of a HTFT process. In that case an iron product was precipitated from a solution containing a dissolved iron salt and the precipitated product (containing certain promoters) was then heat treated under reducing conditions to provide a catalyst with a desired surface area and robustness to be used in a fluidised bed of a HTFT process.
The use of a source of chromium in combination with certain catalysts in certain Fischer-Tropsch reactions and in water-gas shift reactions has been reported in the past. Dry, M E, in “Catalysis-Science and Technology”, Anderson, J R and Boudart, M (eds.) Springer-Verlag, Berlin, 159 (1981), discloses that the addition of Cr2O3 and Al2O3 lowered the catalyst performance of the Co-based LTFT catalyst. Storch, H H, Golumbic, N, Anderson, R B, in “The Fischer-Tropsch and Related Synthesis”, John Wiley and Sons, New York, (1951), discloses the use of Cr2O3 as a promoter in an iron-based catalyst. This reference reports that the presence of Cr2O3 decreased the rate of formation of free carbon in the LTFT process.
Colley S E, Copperthwaite, R G, Hutchings, G J Foulds, G A, Coville, N J, in Appl Catal, 84, 1-15 1992 discloses the addition of chromium to a cobalt-manganese catalyst for a LTFT reaction which resulted in a substantial increase in the selectivity towards C25 to C35 hydrocarbons. In this work, a 20% chromium loading produced an increase in alpha value, as well as an increase in C16+ selectivity from 6.9 to 24.2 mass percent compared to the unpromoted system. This shift towards heavier products was naturally accompanied by a decrease in the yield of light hydrocarbons. The C2 olefin to paraffin ratio was observed to decrease, but the ethylene yield was constant, suggesting that the polymerisation capability of the catalyst was not enhanced. High activity of the CO-catalyst is also mentioned.
Perez, M, Diaz, L, Galinda, H de J, Dominguez, J M, Salmon M; Rev. Soc. Quim. Mex., 43(3,4) 97-99 (1999) is a study of cobalt catalysts wherein a series of Cu—Co—Cr oxides doped with alkali metals (M) was prepared by co-precipitation of metal nitrates and M2CO3 in an aqueous solution. The calcined products were used as catalysts for a LTFT process in a stainless-steel fixed bed microreactor. The composition was chosen with the intention of producing both higher alcohols and hydrocarbons. Methanol, ethanol and 2-propanol were the predominant alcohols formed, and the inclusion of sodium or cerium had the greatest effect on hydrocarbon yield. Chromium was used in this case as an alcohol promoter.
In Zhang, Y, Zhong, B, Wang Q; Cuihua Xuebao, 18 (6), 513-516, (1997) the addition of Cr to a ZrO2—SiO2 supported Co catalyst resulted in lower CO conversion and C5+selectivity, with an increase in methane production in a LTFR process. Similarly Lapidus, A C, Krylova, A Y, Sineva, L V, Durandina, Y V, Motorina, S N; Khim, Tverd. TopL.; (1), 32-38, (1997) discloses that Cr2O3 and alumina decreased the yield of liquid hydrocarbons in a LTFT process.
CN 1140630 discloses a catalyst prepared by co-precipitation and impregnation. This catalyst comprised 80-90% iron oxide, 5.0-15% Cr2O3,1.0-5.0% copper oxide, 0.5-5.0% rare earth oxide (eg cerium oxide), and was suitable for CO conversion via water-gas-shift with suppressed Fischer-Tropsch activity.
The prior art referred to above all relate to LTFT processes, or to a water-gas-shift reaction in the case of CN 1140630, and most refer to Co-based catalysts. From this prior art it is clear that the addition of chromium provided very mixed results. For example in some cases an increased selectivity of heavier hydrocarbons was observed but in other cases the reverse was observed.
It was most surprisingly found that the addition of a source of chromium to an iron based precipitated catalyst resulted in certain advantages when the said precipitate catalyst was used in a HTFT process.